Stereophonic sound recording system



July 15, 1941. R. r. FRIEBUS STEREOPHONIC SOUND RECORDING SYSTEM 3 Sheets-Sheet l Filed March 22, 1938 FIG.

INVENTOR R. T FR/E BUS A TTORNE V July 15,1941. RT, FRIEBUS 2,249,606

STEREOPHONIC SOUND RECORDING SYSTEM Filed March 22, 1938 3 Sheets-Sheet 2 FIG. 4 vc 79 78 we AMP 20 AMP 24 25 AMP MP 2/ 22 vc we 26 Va 27 v; 2&-

AMP {'30 AMP 3/ as LLW 843 I L as INVENTOR R. Z'FR/EBUS ATTORNEY R. T. FRIEBUS 2,249,606

STEREOPHONIC SOUND RECORDING SYSTEM Filed March 22, 1938 July 15, 1941.

3 Shets-Sheet 3 go/iAMP I 1:97 9a:{ AMI FdMP-t-zy 22 9/15 [43-99 loo-Q 5 5%}23 AMP 31 INVENTOR R. 7. F R/E BUS ATTORNEY Patented July 15, 1941 STEREOPHONIC SOUND RECORDING SYSTEM Reginald T. Friebus, Montclair, N. J., assignor to Electrical Research Products, Inc., New York, N. Y., a corporation of Delaware Application March 22, 1938, Serial No. 197,323

8 Claims.

This invention relates to stereophonic sound recording systems and is a continuation in part of application Serial No. 722,475, filed April 26, 1934, now Patent No. 2,114,019, issued April 12, 1938.

Systems capable of producing this type of illusion in general involve the use of two or more spacedsound pick-up devices, a number of spaced receivers and separate circuits connecting the pick-ups with the receivers as fully explained in an article by J. C. Steinberg and W. B. Snow in Electrical Engineering, January 1934." Such systems are intended for and adapted to the purpose of reproducing sounds with a spatial distribution similar to that of the original sounds. The important factors determining the lateral and the fore and aft distribution are'fully eX- plained in the article referred to above and may be briefly summarized as follows:

With a system of this type having two identical channels, the apparent lateral position between the loud-speakers of the sounds corresponding to an actual sound source between the microphones is largely-fixed by the relative strengths of the direct sound waves at the two microphones whereas the fore and aft positions of the reproduced sounds is largely determined by the ratio of the direct sound to the average reverberant sound at the microphone-nearest the actual sound source. sound pick-up purposes have an appreciable reverberationtime so that each sound wave is reflected and re-reflected from opposite walls a sufficient number of times to produce substantially the same intensity of reverberant sound throughout the room. The effective intensity of the direct sound, however, increases rapidly as the source moves toward the pick-up point, so that when the proportion of direct sound in the output of a loud-speaker is increased, the listener hears the sound source as approaching the loudspeaker and when the proportion of direct sound is decreased, the sound source seems to recede from the loud-speaker.

The object of this invention is to record sounds originating'in a sound producing area in such a manner, that when the records are reproduced, the reproduced sounds will have the desired spatial distribution over a reproducing area regardless of the spatial distribution of the original sound sources.

According to the invention, the direct sound energy and the reverberant sound energy supplied to each of several recorders are arbitrarily varied in accordance with the principles outlined In general, rooms suitable for above so that when the records so made are reproduced by means of separate spaced loudspeakers, the reproduced sounds will have the desired spatial distribution. At least two microphones are required. In order to produce apparent motion in the reproduction of a stationary sound source, the microphones are spaced with the source relatively close to one microphone and relatively distant from the other. The output of the close microphone will be largely produced by direct sound waves and the output of the distant microphone, for most rooms, will be largely produced by reflected or reverberant sound so that with proper mixing circuits the proportion of direct to reverberant energy supplied to each receiver and the relative amount of direct sound energy supplied to the several receivers can be varied at will. In this way, a stationary sound source can be recorded and reproduced as a moving source or a moving source can be recorded and reproduced as a stationary source or given any desired apparent motion in the reproduction irrespective of the direction of movement of the actual sound source.

In one embodiment of the invention, two spaced pick-ups and two recorders are used. Each pickup is connected through a separate volume control and unilaterally transmitting device, such as a vacuum tube amplifier, to each of the receivers. The sound source is positioned, or moves in the vicinity of one of the microphones and the several volume controls are operated in the combinations and sequences necessary to produce the desired effects.

Alternatively, in a multichannel stereophonic system of the type described by Steinberg and Snow, each channel may be provided with an independent source of reverberant sound energy, such as a second microphone, disposed at a greater distance from the producing stage than the main pick-up and connected to the channel through a separate amplifier and volume controlling device. With this system, the ratio of direct to reverberant energy can be varied at will in each channel independently of the location of the sound source on the producing stage.

The general procedure outlined above can be used to make records which will produce many varied and fantastic efiects, some of which would be physically impossible to achieve by the actual motion of a real sound source on the recording stage. For example, the reproduction of a large orchestra can be compressed into any desired area, one side of the orchestra or both sides simultaneously can be made to appear to move to the other side and by using an additional distant microphone or by moving one of the microphones in the manner more fully explained below while manipulating the volume controls, the whole orchestra will appear to rotate as a unit.

According to a feature of this invention, the laterally disposed microphones have directional pick-up characteristics in the direction of the scene of action and are disposed to give best results from the standpoint of the desired lateral localization without regard to the effect on the fore and aft distribution of the reproduced sounds. A third microphone disposed at some distance from the stage is connected to both lines through separate amplifiers and controls. The output of this microphone will be largely reverberant sound energy and by controlling the amounts of this energy fed into each line the reproduced sounds may be given any desired fore and aft distribution substantially independently of the acoustics of the particular stage.

In making sound pictures, several cameras disposed at difierent distances from and at different angles with respect to the scene of action, or set, are frequently used and the final picture is made up by combining selected portions of these several takes. In order to record the sounds accompanying such a picture so that they may be reproduced stereophonically, ordinary stereophonic recording technique would require a separate pair of spaced microphones properly disposed with respect to each camera in order to obtain a natural acousticperspective. Such a system, in addition to being expensive, due to the large number of recording channels required, is

also rather unsatisfactory since with a conventional stereophonic pick-up system, the accuracy of localization is rather poor, particularly at the rear of the stage.

According to a further feature of this invention, as applied to the making of stereophonic sound pictures, only .two sound records are made regardless of the number of cameras used. One of these records is made largely from direct sound energy by means of a microphone placed near the set and the other is made largely from .reverberant energy picked up by a microphone at system according to one feature of the invention;

Fig. 2 is a three-channel stereophonic recordin system in which each channel is providedwith both near and distant microphones;

Fig. 3 is a system for recording the sounds accompanying motion pictures;

Fig.4 is a system for making stereophonically related recordings from the records of Fig. 3;

and

Fig. 5 is a system giving for obtaining a desired spacial distribution of reproduced sounds independently of the acoustics of a particular pick-up stage. 7

In the system of Fig. 1 two sound pick-ups or microphones I0 and II spaced in front of the sound producing area or recording stage I2, are connected by separate transmission lines I3, I4 through the power amplifiers 30 and 3| to two recorders 32 and 33 engaging the records 34 and 35 which are driven by the motor 36. The microphones are connected to the corresponding power amplifiers through preliminary amplifiers 20, 2I and volume controlling devices 22 and 23.

' Each microphone is also connected to the other line through similar preliminary amplifiers 24, 25 and volume controls 26, 21. With this arrangement the output of each microphone may be divided between the lines I3 and I4 in any desired proportion. When the volume controls 22, 23 are set for the same transmission efiiciency in each of the lines and the controls 26, 21 are set for zero transmission, 'a moving sound source on the stage I2 will be recorded on the records 34 and 35 in such a manner that when the records are reproduced through spaced loudspeakers disposed before a reproducing stage, the virtual sound source will appear to move on the reproducing stage in a similar manner to the motion of the real source on the stage I2.

Keeping in mind the factors controlling the spacial distribution of the reproduced sound it will be readily understood that this system is capable of making records of such a nature that when reproduced the virtual sound source traces a path on the reproducing stage different from the path of the actual source on the stage I2. Assume, for example, that a singer is standing close to the left-hand microphone I0 and therefore at some distance from the right-hand microphone II. The output current of the microphone II) will be of a distinctly close-up? character, that is to say, the direct sound waves will be relatively stronger than the reflected sound waves reaching the microphone. By the use of volume controls 22 and 26, these currents representing close-up sounds can be admitted into the lines I3, I4 and recorded on the records 34 and 35 in any desired proportion. Likewise, the currents in the output circuit of the microphone II are distinctly distant in characteristic due to the relatively? great distance between this microphone and the singer. By the use of volume controls 2'! and 23 this distant. sound energy can beadmitted to the recorders in any desired proportions. When the volume controls are adjusted so that a large proportion of direct sound energy is fed into the line I3, 2. stereophomc reproduction of the records creates the illusion that the sound source is located at the left-hand side of the reproducing stage. As more of this direct sound energy is admitted into the line I4 the apparent or virtual source moves away from the left side of the stage toward the right in the direct ratio of the proportions of the direct sound admitted to each line. After the voice is brought to the center of the reproducing stage by admitting equal amounts of energy into each line the volume level in line I3 is gradually reduced and the virtual source continues to move to the right and reaches a point close to the right-hand loudspeaker when the energy in line I3 is very low in proportion to the energy in line I4. Similarly, the virtual source can be made to move across the rear of the stage. If an amount of distant sound energy is admitted only into the line I4 the voice appears to be at the rear of the reproducing stage at the right and progresses toward the left rear of the stage as more of this distant sound energy is admitted into the line l3. After the sound source has reached the rear center stage position, the energy level in the line I4 is gradually reduced to zero as before and the source thereupon moves to a position directly behind the left-hand loud-speaker. The sound source may then be moved forward to its starting position by introducing into the line l3 close-up sound energy from the microphone I and at the same time decreasingthe distant energyin this line from the microphone I I.

In the above experiment the simplest procedure for obtaining apparent motion is set forth, but a betterstereophonic illusion will be obtained by manipulating'the volume controls in pairs and in this way the total volume of reproduced sound may be kept at a natural level. When it is necessary to increase either the close-up or the distant energy in one line, a corresponding decrease may be made in the amount of energy of the same type introduced into the other line. Similarly, reductions in the energy level in one line may be compensated by increases in the other line within the limits of the desired variations in the volume of the total sound.

Having in this way delineated the boundaries of the reproducing stage and determined the manner in which the several volume controls must be varied to move the sound source around the stage, the necessary combinations of settings for producing any desired movement within the prescribed area may be readily determined by experiment.

The problem becomes somewhat more complex when a multiple sound source such as an orchestra extending across the whole stage is used. In such a case although there'will be close-up and distant sound energy in both microphone circuits, it is nevertheless possible to move such a sound source laterally 'with the technique already described. For example, the left side of the orchestra may be moved to the right of the reproducing stage and vice verse by simultaneously attenuating the output level of amplifier about decibels from its normal value with volume control 22 and increasing the output level of amplifier 24 from about minus 25 decibels to normal value with volume control 26 and similarly reducing and increasing the levels of the outputs of amplifiers '2! and 25 with the volume controls 23 and 21, respectively. This procedure may be simplified by ganging the controls so that the two controls on each pick-up are operated in pairs. If rotating contact potentiometers are used, for example, the potentiometers of each pair may be connected in opposite polarities and operated by the same shaft. When only one of the pairs of controls is so operated the orchestra will appear to move into a small area at one side of the reproducing stage, but if both, pairs are operated together, the Whole orchestra will appear to have been pulled through itself from left to right and from right to left.

Other interesting and fantastic effects may be obtained by moving one or both of the microphones with respect to the sound source while the controls are being manipulated. For example, the whole orchestra can be made to appear to rotate as a unit in the following manner The volume controls are manipulated in the manner just described to pull the orchestra through itself and then the process is reversed to restore the players to their original relative locations.

For a clockwise rotation' of the orchestra the pick-up I0 is moved slowly forward away from the orchestra and the pick-up H is moved backward closer to the orchestra during the first quarter of the rotation during which the whole orchestra is being concentrated along the center line of the stage. By this procedure the sound from the left side of the orchestra as received by the pick-up l0 gradually changes its characteristic from close-up to distant thereby pulling its sound image to the rear of the reprodu'cing stage While it is being shifted laterally with the control. At the sametime the sound from the right side of the orchestra as received by the pick-up H becomes more and more predominantly close-up in characteristic thereby pulling its sound image forward on the reproducing stage. When the microphones have been moved to their extreme positions as determined'by experiment and the controls are in the mid-positions, the orchestra will appear to have been rotated through degrees. For the second quarter of rotation the microphones are then moved sowly back to their original positions while the volume controlsar'e being operated to their limiting positions in which, as already stated, the outputs of the amplifiers 2t and 2i are attenuated about 25 decibels and the outputs of amplifiers 26 and 21 are restored from about minus 25 decibels to full normal level. The orchestra has then been moved by rotation through degrees to occupy the same position as in the experiment previously described in which the movement was purely lateral. The rotation through the next 180 degrees is accomplished in a similar manner by restoring the volume controls to their original position and simultaneously moving the pick-ups in the reverse manner from that already described, that is, during the first quarter the pick-up is moved slowly backward toward the orchestra and the pick-up H is moved forward away from the orchestra while these controls are returned halfway from their limiting positions toward their original positions. This places the sound image of the orchestra along the center line of the stage. The last quarter of rotation is obtained by gradually restoring the pick-ups and controls to their original positions.

In order to record so as to keep the virtual image of a. large orchestra'within the confines of a reproducing stage of limited depth during the rotating movement, it may sometimes be desirable to rotate only one-half of the orchestra and move the other half laterally across the front of the stage. This is readily accomplished by moving only one'microphone (in the case described the microphone [0) during the operation of the controls.

The fore and aft locations of the microphones with respect to the sound source and the distance through which they must be moved to produce the effect described with best results will, of course, depend upon a number of factors such as the reverberation time of the recording and reproducing studios and the size of the sound source. 7 I

Alternately, the system of Fig. 2 may be used for making stereophonic sound records for later reproduction. In this system, each of the three channels 5|, 52 and 53 is provided with a closeup microphone 60, 6| or 62 and a distant microphone 63, 64 or 65, each of which is connected to the corresponding channel through a separate amplifier and volume controlling device. With a system of this type'in which the proportion'of direct to reverberant energy in each channel may be controlled independently of the other channels, much better perspective or stereophonic efiects may be obtained. I For some purposes it may be desirable to operate each of the six volume controls 54 to 59 separately, but in other cases the two controls on each channel may be operated together as described in connection with the system of Fig. 1. The provision of the center channel in this system, while making it more complex, gives more accurate control of the sound distribution at center stage, particularly in a fore and aft sense.

With this system it will be evident that since each channel has its own source of reverberant sound, no movement of the main microphones 60 to 62 is necessary to produce rotational or other fore and aft movement. For example, in rotating an orchestra instead of moving the microphone 60 farther away, the controls 54 and 55 are operated tofade in a greater proportion of the distant sound energy from the microphone 63 and instead of moving microphone 62 closer to the orchestra, controls 58 and 59 are operated to fade in more energy from microphone 62 and less from the microphone 65.

The outputs of the channels 5|, 52 and 53 may be recorded in any well-known manner such as by recorders 86, 8| and 82 cooperating with a film 83 to make three separate sound tracks. A print from this record may then be reproduced through a three-channel reproducing system having three loud-speakers spaced before a reproducing stage to produce the desired stereophonic sound effects.

In conventional stereophonic pick-up systems the microphones should be located close to the stage in order to get good lateral localization. In many cases, however, particularly when the pick-up stage is relatively "dead this results in a reproduction with so little reverberation as to be unsatisfactory. In such cases the stage acoustics must be improved or the microphones must be placed further back to increase the reverberation at the expense of accuracy in lateral localization.

With the system of Fig. 5 the lateral microphones 9l, 92 may be disposed close to the stage 93 for optimum accuracy of lateral localization substantially independently of the stage acoustics or the desired fore-and-aft distribution. The microphones 9| and 92 are preferably highly directional within the angles a and B projected by the stage at the respective microphones. This directivity of pick-up can be obtained with unidirectional microphones or with bidirectional microphones with baffles 94, 95 disposed behind them or with non-directional microphones equipped with suitable bafiies. As these microphones are close to the stage and are actuated very largely by direct sound waves from the stage the ratio of the energy levels in the two channels will vary in substantially direct proportion to the lateral motions of the sound source. Since lateral localization, as pointed out above, is determined by the ratio of the direct sound energy in the two channels, it will be apparent that this pick-up system gives a high degree ofaccuracy.

By locating a third non-directional microphone 96 at some distance away from the stage so that its output will be very largely reverberant energy and connecting it to both transmitting channels through separate amplifiers 91, 98 and volume controls 99, I00 any desired amount of reverberation can be added to producethe desired artisticeffects. The fore-and-aft localization corresponding to the actual position of the sound source or any other fore-and-aft distribution desired can be obtained by varying the gain settings. For example, by increasing the amount of energy fed into the lines from microphone 96 and decreasing the energyfrom the directional microphones any desired ratio of directto reverberant energy may be obtained and in this way the virtual sound source can be localized as far back on the stage as desired. I

While the lines of this system have been shown as terminating in recorders as in Fig.1, it will be obvious that the pick-up system is equally applicable to systems for direct reproduction and that if desired in a recording system of this type a separate record of the output of microphone 96 may be made and the final'records or reproduction made by reproducing the records and mixing them to produce any spacial dis-. tribution desired in a manner similar to that described below in connection with Figs. 3 and 4.

Since the point at which the reproduced source of sound localizes can be determined at will by a proper proportioning of the direct and reverberant energy in the several channels of the recording system, the invention makes it possible to simplify the procedure and reduce the cost of making stereophonic sound pictures and at the same time to improve the accuracy of localization.

Assume, for example, that a motion picture is being made of the action on the stage 10 of Fig. 3 by means of several cameras H, 12, 13 located at various distances and angles with respect to the. stage in accordance with the usual practice. While the cameras are operating two recordings l4 and 15 are made of the sounds accompanying the scene of action, the record 14 being largely direct sound or sound of minimum liveness due to the close up position of microphone l6 and the record 15 being largely reverberant sound or sound of maximum liveness due to the distant position of microphone 1'! with respect to the stage.

When the final picture has been assembled by combining selected positions of the takes of the several cameras used, the stereophonic sound recordings to accompany the picture are made by means of the system of Fig. 4. The direct and reverberant sound records 14 and 15, or pressings made therefrom, are simultaneously reproduced by the reproducers 16, TI which are connected to an interfading recording system similar to, and having its corresponding parts designated as in the system of Fig. 1.

The volume control 18 is used to determine the amount of reverberant sound energy suitable for the particular picture and the controls 2| and 25 are adjusted until this energy is divided equally between the channels l3 and I4. Once these adjustments are made, they will ordinarily not need to be changed for any given scene, since, as stated above, the random reverberant sound energy is about the same for any location in a room.

The volume control 19 determines the total amount of direct sound energy supplied to the two channels and hence controls the fore-andaft localization obtained when the sound picture is reproduced. The controls 22 and 26 determine the division of the direct energy between the two channels and when operated in opposite directions will produce a lateral movement of the virtual sound when the sound picture is reproduced.

In making up the stereophonic records from the direct and reverberant records M and 15, the final picture record may be projected as a guide to the operator in determining the proper sound localization for each scene. By means of the control 19, he can so re-record that the virtual or reproduced sound source will appear to be in the foreground of the corresponding picture or at any desired distance in the background. By operating the ganged controls 22 and 26, the virtual source will follow the lateral motion of the speaker or other sound source. The changes in the settings of the controls to produce a given apparent motion of the reproduced source will, of course, be first determined experimentally. If desired, loud-speakers 84 and 85 may be bridged across the lines l3, l4 and spaced on opposite sides of the picture screen during the re-recording operation to assist the operator in the operation of the controls.

Since it is common practice in making up a final picture film to delete portions of the original picture record it may be desirable in some cases to make the two original sound records on film rather than on discs, as shown. The film sound records may then be cut to delete those portions corresponding to the deleted picture scenes before re-recording to make the stereophonic records.

While separate recorders cooperating with the same or separate records of any well-known type may be used for making the stereophonic records, it may be desirable in some cases to make the recordings side by side on the same film. British Patent 479,083 discloses a four-ribbon light valve type of recorder for making records of the push-pull type. By connecting one pair of the ribbons of such a recorder 86 to each of the lines [3 and I4, the two stereophonic records 8'! and 88 may be made in the space now used for the sound track of a standard picture film 89.

Properly equipped theatres will then be able to give stereophonic reproduction but others not so equipped can obtain ordinary single channel sound reproduction from the same print by scanning both recordings with their standard single channel reproducing equipment. This type of stereophonic sound picture film will obviously be of great advantage during the period of transition from single channel to stereophonic reproduction.

What is claimed is:

1. The methodof making sound records for the production of stereophonic stage effects which comprises generating currents representing sound waves close to an actual sound source, generating currents representing sound waves at a distance from the source, mixing thecurrents in different proportions according to the desired spacial distribution of the sounds when reproduced, and separately recording said different proportions of mixed currents.

2. The method of making stereophonic sound records which comprises first making records of maximum and minimum, liveness of the sounds, generating currents from the records, mixing the currents in different and varying proportions according to the desired spacial distribution of the sounds when reproduced, and separately recording the different proportions of mixed currents.

3. The method of making stereophonic sound records which comprises first making records of maximum and minimum liveness of the sounds, generating currents representing selected portions of the record, mixing the currents in different and varying proportions according to the spacial distribution desired, and making a plurality of records of the mixed currents.

4. The method of recording the sound for stereophonic sound pictures consisting of a sequence of scenes made by cameras variously disposed with respect to the scene of action which comprises making two preliminary recordings of the sounds accompanying the scene of action, one of the records containing more reverberation than the other, generating currents corresponding to the recordings, mixing the currents in difierent proportions according to the spacial distribution of the sound sources in the corresponding scene of the picture, and separately recording the different proportions of mixed currents.

5. The method of making stereophonically related sound records for sound pictures which comprises making two records of the sounds accompanying the picture scene, one of the records containing more reverberation than the other, generating currents from the records, viewing a projection of the picture, and mixing portions of the currents from the records in different proportions simultaneously as required to localize the sounds in the positions of the sound sources shown in the picture and making a plurality of other records of said mixed currents.

6. In a recording system, two sources of current representing the same sounds with different amounts of reverberation, two recording devices, a unilaterally conducting circuit between each source and each recording device, and means for controlling the energy level in each of said circuits.

7. In a stereophonic recording system, a plurality of sound pick-up means disposed in spaced relation close to a sound producing area, each of said means having a highly directional sensitivity for sounds originating in the area, a separate transmitting channel for each pick-up means, other pick-up means disposed at a distance from the area and separate circuits each including a volume control connecting the other pickup means to both of the channels.

8. The method of making stereophonically related sound records which comprises separately generating currents representing direct sound waves from a sound source at a plurality of fixed spaced points chosen with respect'to the sound source to give optimum lateral localization, generating currents representing reverberant sound from said source, separately mixing the reverber ant currents with each of the other currents in accordance with the desired reverberation and fore-and-aft localization and separately recording the plurality of mixed currents.

REGINALD T. FRIEBUS. 

